PROBLEM TO BE SOLVED: To provide a bearing assembly cam shaft capable of more effectively attaining reduction in manufacturing cost.SOLUTION: In the bearing assembly cam shaft including a cam shaft body (10) having a plurality of steps of cylindrical parts (101, 102, 103) whose diameters are sequentially made small toward one end side or both end sides and cam pieces (30) and bearings (20) which are attached to the plurality of steps of cylindrical parts of the cam shaft body, the bearing (20) is a rolling bearing including an inner ring (22) and an outer ring (24) and a rolling bodies (28) disposed between the inner ring and the outer ring and retained in a retainer (26) and the inner ring (22) of the rolling bearing has the inner diameter adapted to the respective outer diameters of the plurality of steps of cylindrical parts and the rolling bodies retained by the outer ring and the retainer are the same in the respective parts of the plurality of steps of the cylindrical parts.